1. Field of the Invention
The present invention relates to a transformer having at least one high voltage winding and one low voltage winding. The invention is applicable to power transformers having rated outputs from a few hundred kVA to more than 1000 MVA and rated voltages from 3-4 kV to very high transmission voltages, e.g. from 400-800 kV or higher.
2. Discussion of the Background
Conventional power transformers are described in, e.g., A. C. Franklin and D. P. Franklin, “The & Transformer Book, A Practical Technology of the Power Transformer”, published by Butterworths, 11th edition, 1990. Problems related to internal electric insulation and related topics are discussed in, e.g., H. P. Moser, “Transformerboard, Die Verwendung von Transformerboard in Grossleistungstransformatoren”, published by H. Weidman AG, Rapperswil mit Gesamtherstellung: Birkhäuser AG, Basle, Switzerland.
In transmission and distribution of electric energy transformers are exclusively used for enabling exchange of electric energy between two or more electric systems. Transformers are available for powers from the 1 VA region to the 1000 MVA region and for voltages up to the highest transmission voltages used today.
Conventional power transformers include a transformer core, often formed of laminated commonly oriented sheet, normally of silicon iron. The core is formed of a number of legs connected by yokes which together form one or more core windows. Transformers having such a core are usually called core transformers. A number of windings are provided around the core legs. In power transformers, these windings are almost always arranged in a concentric configuration and distributed along the length of the core leg.
Other types of core structures are, however, known, e.g. so-called shell transformer structures, which normally have rectangular windings and rectangular leg sections disposed outside the windings.
Air-cooled conventional power transformers for lower power ranges are known. To render these transformers screen-protected an outer casing is often provided, which also reduces the external magnetic fields from the transformers.
Most power transformers are, however, oil-cooled the oil also serving as an insulating medium. An oil-cooled and oil-insulated conventional transformer is enclosed in an outer case which has to fulfil heavy demands. The construction of such a transformer with its associated circuit couplers, breaker elements and bushings is therefore complicated. The use of oil for cooling and insulation also complicates service of the transformer and constitutes an environmental hazard.
A so called “dry” transformer without oil insulation and oil cooling and adapted for rated powers up to 1000 MVA with rated voltages from 3-4 kV and up to very high transmission voltages has windings formed from conductors such as shown in FIG. 1. The conductor has a central conductor composed of a number of non-insulated (and optionally some insulated) wire strands 5 and 6000 respectively. Around the conductor there is an inner semiconducting casing 6 which is in contact with at least some of the non-insulated strands 5. This semiconducting casing 6 is in turn surrounded by the main insulation of the cable in the form of an extruded solid insulating layer 7. This insulating layer 7 is surrounded by an external semiconducting casing 8. The conductor area of the cable can vary between 80 and 3000 mm2 and the external diameter of the cable between 20 and 250 mm. A metal shield 500 and sheath 5000 surround the external semiconducting casing 8, as shown.
Whilst the casings 6 and 8 are described as “semiconducting” they are in practice formed from a base polymer mixed with carbon black or metallic particles and have a resistivity of between 1 and 105 Ωcm, preferably between 10 and 500 Ωcm. Suitable base polymers for the casings 6 and 8 (and for the insulating layer 7) include ethylene vinyl acetate copolymer/nitrile rubber, butyl grafted polythene, ethylene butyl acrylate copolymer, ethylene ethyl acrylate copolymer, ethylene propene rubber, polyethylenes of low density, poly butylene, poly methyl pentene, and ethylene acrylate copolymer.
The inner semiconducting casing 6 is rigidly connected to the insulating layer 7 over the entire interface therebetween. Similarly, the outer semiconducting casing 8 is rigidly connected to the insulating layer 7 over the entire interface therebetween. The casings 6 and 8 and the layer 7 form a solid insulation system and are conveniently extruded together around the wire strands 5.
Whilst the conductivity of the inner semiconducting casing 6 is lower than that of the electrically conductive wire strands 5, it is still sufficient to equalise the potential over its surface. Accordingly, the electric field is distributed uniformly around the circumference of the insulating layer 7 and the risk of localised field enhancement and partial discharge is minimised.
The potential at the outer semiconducting casing 8, which is conveniently at zero or ground or some other controlled potential, is equalised at this value by the conductivity of the casing. At the same time, the semiconducting casing 8 has sufficient resistivity to enclose the electric field. In view of this resistivity, it is desirable to connect the conductive polymeric casing to ground, or some other controlled potential, at intervals therealong.
The transformer according to the invention can be a one-, three- or multi-phase transformer and the core can be of any design. FIG. 2 shows a three-phase laminated core transformer. The core is of conventional design and includes three core legs 9, 10, 11 and joining yokes 12, 13.
The windings are concentrically wound around the core legs. In the transformer of FIG. 2 there are three concentric winding turns 14, 15, 16. The innermost winding turn 14 can represent the primary winding and the two other winding turns 15,16 the secondary winding. To make the Figure more clear such details as connections for the windings are left out. Spacing bars 17,18 are provided at certain locations around the windings. These bars 17,18 can be made of insulating material to define a certain space between the winding turns 14, 15, 16 for cooling, retention etc. or be made of an electrically conducting material to form a part of a grounding system of the windings 14, 15, 16.
The mechanical design of the individual coils of a transformer must be such that they can withstand forces resulting from short circuit currents. As these forces can be very high in a power transformer, the coils must be distributed and proportioned to give a generous margin of error and for that reason the coils cannot be designed so as to optimize performance in normal operation.